Transparent conducting oxide for photovoltaic devices

ABSTRACT

One embodiment of the present invention provides a solar cell. The solar cell includes a Si base layer, a passivation layer situated above the Si base layer, a layer of heavily doped amorphous Si (a-Si) situated above the passivation layer, a first transparent-conducting-oxide (TCO) layer situated above the heavily doped a-Si layer, a back-side electrode situated below the Si base layer, and a front-side electrode situated above the first TCO layer. The first TCO layer comprises at least one of: GaInO, GaInSnO, ZnInO, and ZnInSnO.

RELATED APPLICATION

This application is a continuation application of application Ser. No.13/155,112, Attorney Docket Number SCTY-P58-2NUS, entitled “TRANSPARENTCONDUCTING OXIDE FOR PHOTOVOLTAIC DEVICES,” by inventors Jianming Fu,Zheng Xu, Jiunn Benjamin Heng, and Chentao Yu, filed 7 Jun. 2011, whichclaims the benefit of U.S. Provisional Application No. 61/353,119,Attorney Docket Number SSP10-1009PSP, entitled “Transparent ConductingOxide for Photovoltaic Devices,” by inventors Jianming Fu, Zheng Xu,Jiunn Benjamin Heng, and Chentao Yu, filed 9 Jun. 2010.

BACKGROUND

1. Field

This disclosure is generally related to solar cells. More specifically,this disclosure is related to a solar cell that includes a high workfunction transparent conducting oxide (TCO) layer.

2. Related Art

The negative environmental impact caused by the use of fossil fuels andtheir rising cost have resulted in a dire need for cleaner, cheaperalternative energy sources. Among different forms of alternative energysources, solar power has been favored for its cleanness and wideavailability.

A solar cell converts light into electricity using the photoelectriceffect. There are several basic solar cell structures, including asingle p-n junction, p-i-n/n-i-p, and multi-junction. A typical singlep-n junction structure includes a p-type doped layer and an n-type dopedlayer. Solar cells with a single p-n junction can be homojunction solarcells or heterojunction solar cells. If both the p-doped and n-dopedlayers are made of similar materials (materials with equal band gaps),the solar cell is called a homojunction solar cell. In contrast, aheterojunction solar cell includes at least two layers of materials ofdifferent bandgaps. A p-i-n/n-i-p structure includes a p-type dopedlayer, an n-type doped layer, and an intrinsic (undoped) semiconductorlayer (the i-layer) sandwiched between the p-layer and the n-layer. Amulti-junction structure includes multiple single-junction structures ofdifferent bandgaps stacked on top of one another.

In a solar cell, light is absorbed near the p-n junction generatingcarriers. The carriers diffuse into the p-n junction and are separatedby the built-in electric field, thus producing an electrical currentacross the device and external circuitry. An important metric indetermining a solar cell's quality is its energy-conversion efficiency,which is defined as the ratio between power converted (from absorbedlight to electrical energy) and power collected when the solar cell isconnected to an electrical circuit.

For homojunction solar cells, minority-carrier recombination at the cellsurface due to the existence of dangling bonds can significantly reducethe solar cell efficiency; thus, a good surface passivation process isneeded. In addition, the relatively thick, heavily doped emitter layer,which is formed by dopant diffusion, can drastically reduce theabsorption of short wavelength light. Comparatively, heterojunctionsolar cells, such as Si heterojunction (SHJ) solar cells, areadvantageous. FIG. 1 presents a diagram illustrating an exemplary SHJsolar cell (prior art). SHJ solar cell 100 includes front fingerelectrode 102, a heavily doped amorphous-silicon (a-Si) emitter layer104, an intrinsic a-Si layer 106, a crystalline-Si substrate 108, and anAl back-side electrode 110. Arrows in FIG. 1 indicate incident sunlight.Because there is an inherent bandgap offset between a-Si layer 106 andcrystalline-Si (c-Si) layer 108, a-Si layer 106 can be used to reducethe surface recombination velocity by creating a barrier for minoritycarriers. The a-Si layer 106 also passivates the surface ofcrystalline-Si layer 108 by repairing the existing Si dangling bonds.Moreover, the thickness of heavily doped a-Si emitter layer 104 can bemuch thinner compared to that of a homojunction solar cell. Thus, SHJsolar cells can provide a higher efficiency with higher open-circuitvoltage (V_(oc)) and larger short-circuit current (J_(sc)).

When fabricating solar cells, a layer of transparent conducting oxide(TCO) is often deposited on the a-Si emitter layer to form anohmic-contact. However, due to the large band gap and high work functionof the heavily doped p⁺ amorphous Si emitter layer, it is hard to formlow-resistance ohmic contact between a conventional TCO material, suchas indium tin oxide (ITO), and the heavily doped a-Si emitter.

SUMMARY

One embodiment of the present invention provides a solar cell. The solarcell includes a Si base layer, a passivation layer situated on a firstside of the Si base layer, a layer of heavily doped p-type amorphoussemiconductor situated on the passivation layer, a firsttransparent-conducting-oxide (TCO) layer situated on the heavily dopedamorphous semiconductor layer, and a first electrode situated on thefirst TCO layer. The first TCO layer comprises at least one of: GaInO,GaInSnO, ZnInO, and ZnInSnO.

In a variation on the embodiment, the first side of the Si base layer isfacing the incident sunlight.

In a variation on the embodiment, the solar cell includes a secondelectrode situated on a second side of the Si base layer, and the secondside is opposite to the first side.

In a further variation, the second side of the Si base layer is facingthe incident sunlight, and the second electrode includes a second TCOlayer and a metal grid comprising Cu and/or Ni.

In a variation on the embodiment, the Si base layer includes acrystalline-Si (c-Si) substrate.

In a variation on the embodiment, the Si base layer includes anepitaxially formed crystalline-Si (c-Si) thin film.

In a variation on the embodiment, the passivation layer includes atleast one of: undoped a-Si and SiO_(x).

In a variation on the embodiment, the heavily doped p-type amorphoussemiconductor layer has a doping concentration between 1×10¹⁷/cm³ and5×10²⁰/cm³.

In a variation on this embodiment, the first TCO layer has a workfunction between 4.9 eV and 6.1 eV.

In a variation on the embodiment, the solar cell further comprises athird TCO layer situated on the first TCO layer, and the third TCO layerhas a lower resistivity than the first TCO layer.

In a further variation, the third TCO layer includes at least one of:indium tin oxide (ITO), tin-oxide (SnOx), aluminum doped zinc-oxide(ZnO:Al), and Ga doped zinc-oxide (ZnO:Ga).

In a variation on the embodiment, the first electrode comprises at leastone of: Ag, Cu, and Ni.

In a variation on the embodiment, the p-type amorphous semiconductorcomprises amorphous Si or amorphous Si containing carbon.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 presents a diagram illustrating an exemplary Si heterojunction(SHJ) solar cell (prior art).

FIG. 2 presents a diagram illustrating the band diagrams at theinterface between high/medium/low work function TCO material and p-typeamorphous Si.

FIG. 3 presents a diagram illustrating the process of fabricating asolar cell in accordance with an embodiment of the present invention.

FIG. 4 presents a diagram illustrating an exemplary solar cell inaccordance with an embodiment of the present invention.

FIG. 5 presents a diagram illustrating an exemplary solar cell inaccordance with an embodiment of the present invention

In the figures, like reference numerals refer to the same figureelements.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the embodiments, and is provided in the contextof a particular application and its requirements. Various modificationsto the disclosed embodiments will be readily apparent to those skilledin the art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present disclosure. Thus, the present invention is notlimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein.

Overview

Embodiments of the present invention provide an SHJ solar cell thatincludes a layer of novel TCO material with high work function. Therelatively high work function, up to 6.1 eV, of the TCO material ensureslower contact resistance and higher V_(oc).

TCO film has been widely used in solar cells to form ohmic contact withthe emitter layer. An SHJ solar cell can be formed by depositing a-Silayers on a c-Si substrate. Note that the a-Si layers include a layer ofdoped a-Si in order to form a junction with the c-Si substrate or toensure good electrical contact with a subsequently formed electrode. ATCO layer is often deposited on the doped a-Si layer to form an ohmiccontact. However, due to the large band gap and high work function ofthe p-type doped a-Si layer, it is difficult to find a TCO material withwork function that is in alignment with the p-type a-Si in order tominimize the band bending at the TCO and p-type a-Si interface, and toreduce contact resistance and maximize open circuit voltage. Forexample, the work function of ITO is between 4.5 eV and 4.8 eV. Thiswill cause band bending at TCO and p-type a-Si interface, and make ithard to achieve a low-resistance ohmic contact and high V_(oc). FIG. 2presents a diagram illustrating the band diagrams at the interfacebetween high/medium/low work function TCO material and p-type amorphousSi. From the band diagram, one can see that, for TCO material with lowor medium work function, potential barriers at the interface make itharder for charges (holes) to migrate from the p-type a-Si material tothe TCO, thus resulting in higher contact resistance. Hence, it isdesirable to use a TCO material that has a relatively high workfunction.

FIG. 3 presents a diagram illustrating the process of fabricating asolar cell in accordance with an embodiment of the present invention.

In operation 3A, a substrate 300 is prepared. In one embodiment,substrate 300 is a c-Si substrate, which is textured and cleaned. C-Sisubstrate 300 can be either p-type doped or n-type doped. In oneembodiment, c-Si substrate 300 is lightly doped with an n-type dopant,and the doping concentration of c-Si substrate 300 can be between1×10¹⁶/cm³ and 1×10¹⁷/cm³. Note that other than using c-Si substrate(which is more expensive) as a base layer, it is also possible todeposit a thin c-Si epitaxial film on a relatively cheapermetallurgical-grade Si (MG-Si) substrate to act as a base layer, thuslowering the manufacturing cost. The thickness of the c-Si epitaxialfilm can be between 5 μm and 100 μm. The surface of c-Si substrate 300can be textured to maximize light absorption inside the solar cell, thusfurther enhancing efficiency. The surface texturing can be performedusing various etching techniques including dry plasma etching and wetetching. The etchants used in the dry plasma etching include, but arenot limited to: SF₆, F₂, and NF₃. The wet etching etchant can be analkaline solution. The shapes of the surface texture can be pyramids orinverted pyramids, which are randomly or regularly distributed on thesurface of c-Si substrate 300.

In operation 3B, a passivation layer 304 is deposited on top of c-Sisubstrate 300. Passivation layer 304 can significantly reduce thedensity of surface carrier recombination, thus increasing the solar cellefficiency. Passivation layer 304 can be formed using differentmaterials such as intrinsic a-Si or silicon-oxide (SiO_(x)). In oneembodiment, a layer of intrinsic a-Si is deposited on c-Si substrate 300to form passivation layer 304. Techniques used for forming passivationlayer 304 include, but are not limited to: PECVD, sputtering, andelectron beam (e-beam) evaporation. The thickness of passivation layer304 can be between 3 nm and 10 nm.

In operation 3C, a heavily doped p-type doped amorphous semiconductorlayer is deposited on passivation layer 304 to form an emitter layer306. The p-type amorphous semiconductor can be a-Si or amorphous SiC(a-SiC). In one embodiment, emitter layer 306 includes a-Si. The dopingconcentration of emitter layer 306 can be between 1×10¹⁷/cm³ and5×10²⁰/cm³. The thickness of emitter layer 306 can be between 3 nm and10 nm. Techniques used for depositing emitter layer 306 include PECVD.Because the thickness of emitter layer 306 can be much smaller comparedwith that of the emitter layer in a homojunction solar cell, theabsorption of short wavelength light is significantly reduced, thusleading to higher solar cell efficiency.

In operation 3D, a layer of high work function TCO material is depositedon top of emitter layer 306 to form TCO layer 308. Compared withconventional TCO material, such as ITO, used in solar cells, TCO layer308 includes TCO material with a relatively higher work function. In oneembodiment, the work function of TCO layer 308 is between 4.9 eV and 6.1eV. Examples of high work function TCO include, but are not limited to:GaInO (GIO), GaInSnO (GITO), ZnInO (ZIO), ZnInSnO (ZITO), theircombinations, as well as their combination with ITO. Techniques used forforming TCO layer 308 include, but are not limited to: PECVD,sputtering, and e-beam evaporation. Note that in addition to providinglow-resistance ohmic contact, the higher work function of TCO layer 308can also result in a higher V_(oc).

In operation 3E, metal front electrodes 310 are formed on top of TCOlayer 308. Front metal electrodes 310 can be formed using various metaldeposition techniques at a low temperature of less than 300° C. In oneembodiment, front electrodes 310 are formed by screen-printing Ag paste.In another embodiment, front electrodes 310 are formed by electroplatingCu and/or Ni.

In operation 3F, a back electrode 302 is formed on the opposite side tothe front side. In one embodiment, the back electrode stack can includea passivation layer, an n-typed heavily doped semiconductor layer, a TCOor a metal layer with relatively low work function (such as between 4.0eV and 5.0 eV), and a metal grid.

After the formation of front electrodes 310 and back electrode 302,various techniques such as laser scribing can be used for cell isolationto enable series interconnection of solar cells.

Although adopting high work function TCO material can result in lowercontact resistance between TCO layer 308 and emitter layer 306, highwork function TCO material tends to have a larger resistivity than thatof the ITO. For example, an ITO material that has 5% tin oxide has a lowresistivity of 200 μΩ·cm, which is much smaller than that of the highwork function TCO materials. Hence, to reduce the overall resistance,TCO layer 308 may be a bi-layer structure that includes a high workfunction TCO sub-layer and an ITO sub-layer.

FIG. 4 presents a diagram illustrating an exemplary solar cell inaccordance with an embodiment of the present invention. Solar cell 400includes a base layer 402, a passivation layer 404, an emitter layer406, a TCO layer 408, a back-side electrode 410, and a front-side metalgrid 412.

Base layer 402 can be a c-Si substrate or an epitaxially formed c-Sithin film. Passivation layer 404 can be an oxide layer or a layer ofintrinsic a-Si. Emitter layer 406 can be either p-type doped or n-typedoped. In one embodiment, emitter layer 406 is p-type doped a-Si. TCOlayer 408 includes two sub-layers 408-1 and 408-2. Sub-layer 408-1 is ontop of emitter layer 406. To ensure a good ohmic contact with a lowcontact resistance, in one embodiment, sub-layer 408-1 is formed usinghigh work function TCO material, including, but not limited to: GaInO(GIO), GaInSnO (GITO), ZnInO (ZIO), ZnInSnO (ZITO), and theircombinations. Sub-layer 408-2 includes TCO materials having lowresistivity, such as ITO, tin-oxide (SnO_(x)), aluminum doped zinc-oxide(ZnO:Al), or Ga doped zinc-oxide (ZnO:Ga). Back-side electrode caninclude a passivation layer, an n-typed heavily doped semiconductorlayer, a TCO or a metal layer with relatively low work function (such asthat between 4.0 eV and 5.0 eV), and a metal grid. Front-side metal grid412 can include screen-printed Ag grid or electroplated Cu and/or Nigrid.

In addition to be deposited on the front side (the side facing the sun)of the solar cell, the high work function TCO layer can also be used onthe side opposite to the incidence of sunlight. In one embodiment, thepassivation layer and the heavily doped p-type semiconductor layer aredeposited on the back side of the c-Si base layer, facing away fromincident light. The high work function TCO layer is then deposited onthe back side as well. The electrode on the front side of the solar cellincludes a TCO layer with lower work function, such as ITO. The solarcell performance can still benefit from the low ohmic contact resistancebetween the high-work function TCO and the heavily doped p-typesemiconductor layer.

FIG. 5 presents a diagram illustrating an exemplary solar cell inaccordance with an embodiment of the present invention. Solar cell 500includes a base layer 502, passivation layers 504 and 506, an emitterlayer 508, a BSF layer 510, TCO layers 512 and 514, a back-sideelectrode 516, and a front-side electrode 518.

Base layer 502 can be lightly doped c-Si. In one embodiment, base layer502 is p-type doped. Passivation layers 504 and 506 can include anintrinsic a-Si or oxide layer or a combination thereof. Emitter layer508 can be heavily doped n-type amorphous semiconductor, and BSF layer510 can be heavily doped p-type amorphous semiconductor, such as a-Si ora-SiC. Front-side TCO layer 512 interfaces with n-type doped emitterlayer 508, and includes low work function TCO material, such as ITO.Back-side TCO layer 514 interfaces with p-type doped BSF layer 510, andincludes high work function TCO material, such as GIO, GITO, ZIO, ZITO,and their combinations. Back-side electrode 516 and front-side electrode518 are similar to the ones shown in FIG. 4.

Note that it is also possible to place the heavily doped p-type emitteron the back side of the solar cell with a lightly doped n-type baselayer, and to include a front surface field (FSF) layer. As long as theTCO material interfacing with heavily doped p-type material has arelatively high work function, the overall performance of the solar cellcan benefit from the reduced ohmic contact resistance between the TCOand the heavily doped p-type material.

The foregoing descriptions of various embodiments have been presentedonly for purposes of illustration and description. They are not intendedto be exhaustive or to limit the present invention to the formsdisclosed. Accordingly, many modifications and variations will beapparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present invention.

What is claimed is:
 1. A photovoltaic structure, comprising: a Si baselayer; a p-type doped semiconductor layer positioned on a first side ofthe Si base layer; an n-type doped semiconductor layer positioned on asecond side of the Si base layer; a first transparent-conductive-oxidelayer positioned on and in direct contact with the p-type dopedamorphous semiconductor layer, wherein the firsttransparent-conductive-oxide layer has a work function that is between4.9 eV and 6.1 eV; and a second transparent-conductive-oxide layerpositioned on and in direct contact with the n-type doped amorphoussemiconductor layer, wherein the second transparent-conductive-oxidelayer has a work function that is lower than the work function of thefirst transparent-conductive-oxide layer.
 2. The photovoltaic structureof claim 1, further comprising a first metallic grid positioned on thefirst transparent-conductive-oxide layer, wherein the first metallicgrid comprises an electroplated layer that includes Cu.
 3. Thephotovoltaic structure of claim 1, further comprising a second metallicgrid positioned on the second transparent-conductive-oxide layer,wherein the second metallic grid comprises an electroplated layer thatincludes Cu.
 4. The photovoltaic structure of claim 1, wherein the Sibase layer includes a crystalline-Si substrate or an epitaxially formedcrystalline-Si thin film.
 5. The photovoltaic structure of claim 1,further comprising at least one of: a first passivation layer positionedbetween the Si base layer and the p-type doped semiconductor layer; anda second passivation layer positioned between the Si base layer and thep-type doped semiconductor layer.
 6. The photovoltaic structure of claim5, wherein the first passivation layer and the second passivation layerinclude at least one of: undoped amorphous Si and SiO_(x).
 7. Thephotovoltaic structure of claim 1, wherein the p-type dopedsemiconductor layer comprises amorphous Si or amorphous Si containingcarbon, and wherein the p-type doped semiconductor layer has a dopingconcentration between 1×10¹⁷/cm³ and 5×10²⁰/cm³.
 8. The photovoltaicstructure of claim 1, further comprising a thirdtransparent-conductive-oxide layer positioned on the firsttransparent-conductive-oxide layer, wherein the thirdtransparent-conductive-oxide is selected from a group consisting of:indium tin oxide (ITO), tin-oxide (SnO_(x)), aluminum doped zinc-oxide(ZnO:Al), Ga doped zinc-oxide (ZnO:Ga), and any combination thereof. 9.The photovoltaic structure of claim 1, wherein the firsttransparent-conductive-oxide layer is selected from a group consistingof: GaInO (GIO); GaInSnO (GITO); ZnInO (ZIO); ZnInSnO (ZITO); and anycombination thereof.
 10. The photovoltaic structure of claim 1, whereinthe second transparent-conductive-oxide layer comprises indium tin oxide(ITO).
 11. A method for fabricating a photovoltaic structure,comprising: preparing a Si base layer; forming a p-type dopedsemiconductor layer positioned on a first side of the Si base layer;forming an n-type doped semiconductor layer positioned on a second sideof the Si base layer; depositing a first transparent-conductive-oxidelayer on a surface of the p-type doped amorphous semiconductor layer,wherein the first transparent-conductive-oxide layer has a work functionthat is between 4.9 eV and 6.1 eV; and depositing a secondtransparent-conductive-oxide layer on a surface of the n-type dopedamorphous semiconductor layer, wherein the secondtransparent-conductive-oxide layer has a work function that is lowerthan the work function of the first transparent-conductive-oxide layer.12. The method of claim 11, further comprising forming a first metallicgrid on a surface of the first transparent-conductive-oxide layer,wherein the first metallic grid comprises an electroplated layer thatincludes Cu.
 13. The method of claim 11, further comprising forming asecond metallic grid on a surface of the secondtransparent-conductive-oxide layer, wherein the second metallic gridcomprises an electroplated layer that includes Cu.
 14. The method ofclaim 11, wherein preparing the Si base layer involves obtaining acrystalline-Si substrate or epitaxially forming a crystalline-Si thinfilm.
 15. The method of claim 11, further comprising at least one of:forming a first passivation layer positioned between the Si base layerand the p-type doped semiconductor layer; and forming a secondpassivation layer positioned between the Si base layer and the p-typedoped semiconductor layer.
 16. The method of claim 15, wherein the firstpassivation layer and the second passivation layer include at least oneof: undoped amorphous Si and SiO_(x).
 17. The method of claim 11,wherein the p-type doped semiconductor layer comprises amorphous Si oramorphous Si containing carbon, and wherein the p-type dopedsemiconductor layer has a doping concentration between 1×10¹⁷/cm³ and5×10²⁰/cm³.
 18. The method of claim 11, further comprising forming athird transparent-conductive-oxide layer on the firsttransparent-conductive-oxide layer, wherein the thirdtransparent-conductive-oxide is selected from a group consisting of:indium tin oxide (ITO), tin-oxide (SnOx), aluminum doped zinc-oxide(ZnO:Al), Ga doped zinc-oxide (ZnO:Ga), and any combination thereof. 19.The method of claim 11, wherein the first transparent-conductive-oxidelayer is selected from a group consisting of: GaInO (GIO); GaInSnO(GITO); ZnInO (ZIO); ZnInSnO (ZITO); and any combination thereof. 20.The method of claim 11, wherein the second transparent-conductive-oxidelayer comprises indium tin oxide (ITO).